1. Field of the Invention
This invention relates to an apparatus and method for the controlled drying of plastic pellets that are ultimately melted and utilized to form plastic articles of manufacture. The controlled drying of the plastic pellets provides energy savings in the drying process and the properly dried pellet produces a better article of manufacture.
2. Description of the Prior Art
Dryers and dryer hoppers have been utilized in the plastics industry for many years. These dryers provide heated, dehumidified air to a dryer hopper. Plastic pellets within the dryer hopper are exposed to the heated, dehumidified air to remove moisture from the pellets. The basic purpose of drying the plastic pellets is to remove moisture from the surface and interior of the pellets before the pellets reach the melt phase as they pass through the screw area of an injection or extrusion machine. If the moisture is not removed, processing problems may occur or the quality of the finished parts may be inferior to parts which were made with properly dried material. Some properties of the parts which may be affected by inadequate drying are: tensile strength; impact strength; surface blemishes; and degradation of barrier properties. Moisture may be either on the surface of the plastic pellet or in the polymer chain itself. Hygroscopic resin absorbs moisture into polymer chain itself. Other resins hold only surface moisture. Proper heating and dehumidifying of the pellet causes the moisture to be driven out of hygroscopic resins onto the surface. Surface moisture is removed by the heated dehumidified air.
FIG. 1, which is prior art, shows a typical dryer and dryer hopper arrangement for drying plastic pellets. An insulated dryer hopper 10 is provided that has a cylindrical body 12 with a frusto conical lower body portion 14. An outlet 16 is provided at the bottom of the frusto conical lower body. At the top of the dryer hopper 10, a pellet loader 18 is typically provided to intermittently load pellets into the dryer hopper 10. The pellets move downwardly through the dryer hopper 10 in the direction of the arrows shown within hopper 10.
The dryer hopper 10 has a spreader cone 20, a heated air inlet 22 and an air outlet 24. The heated air within hopper 10 passes upwardly through the bed of pellets from inlet 22 to outlet 24 in a direction opposite the movement of pellets within the hopper.
Air from air outlet 24 typically passes through an after cooler 26 to cool the air after it leaves the dryer hopper 10. The cooled air then enters a process filter 28 to remove any dust and fines that may be entrained in the air system. Air from the process filter 28 enters blower 30 which provides a constant air flow through the drying circuit. The air then travels to the desiccant section 32 where moisture is removed from the air. The typical desiccant section 32 contains molecular sieves usually formed of Alumino-Silicates which remove the moisture from the air entering the desiccant section 32. The air then travels through the process heater 34 where it is heated to a temperature sufficient to dry pellets within the dryer hopper 10. A valve 35 controls the flow of heated air from the heater 34 to the dryer hopper 10. The process filter 28, blower 30, desiccant section 32 and process heater 34 are typically located within a dryer unit 36 that is positioned adjacent to the dryer hopper 10.
As the heated air enters the dryer hopper 10 through heated air inlet 22, it passes upwardly through the bed of pellets and the heat from the air is transferred to the pellets causing the release of moisture which is then carried to the top of the hopper with the air to air outlet 24 and the cycle is continued. This closed loop cycle continues throughout the drying process.
Efforts have been made to control the drying process within the dryer hopper 10 by controlling the temperature of the exhaust air from the hopper. U.S. Pat. No. 4,413,426 is an example of the such an arrangement.
The present invention is directed to modification of the dryer hopper 10 which will provide vertically spaced temperature sensors throughout the dryer hopper from the bottom to the top of the hopper and provide control of drying process utilizing data obtained from the vertically spaced temperature sensors.